On Thu, 14 September 2006 11:31:23 +0200, Jörn Engel wrote:
>
> After taking a look at struct dentry, Arnd noted an alignment
> problem. The first four fields currently are:
> atomic_t d_count;
> unsigned int d_flags; /* protected by d_lock */
> spinlock_t d_lock; /* per dentry lock */
> struct inode *d_inode; /* Where the name belongs to - NULL is
> * negative */
> On 64bit architectures, the first three take 12 bytes and d_inode is
> not naturally aligned, so it can be aligned to byte 16. This grows a
> struct dentry from 196 to 200 Bytes (assuming no funky config options
> like DEBUG_*, PROFILING or PREEMT && SMP are set).
>
> One possible solution would be to exchange d_inode with d_mounted, but
> I fear that d_inode would move from a hot cacheline to a cold one,
> reducing performance. Could there be a good solution or would any
> rearrangement here only cause regressions?
>
> Also, both 196 and 200 bytes are fairly close to 192 bytes, so I could
> imagine performance improvements on 64bit machines with 64 Byte
> cachelines. Might it make sense to trim DNAME_INLINE_LEN_MIN by 4 or
> 8 bytes for such machines?
And here is a patch for those who prefer talking code
Jörn
--
Joern's library part 8:
http://citeseer.ist.psu.edu/plank97tutorial.html
--- slab/include/linux/dcache.h~dentry_alignment 2006-09-14 10:52:51.000000000 +0200
+++ slab/include/linux/dcache.h 2006-09-14 12:44:56.000000000 +0200
@@ -77,14 +77,17 @@ full_name_hash(const unsigned char *name
struct dcookie_struct;
+#if BITS_PER_LONG == 64
+#define DNAME_INLINE_LEN_MIN 32
+#else
#define DNAME_INLINE_LEN_MIN 36
+#endif
struct dentry {
atomic_t d_count;
unsigned int d_flags; /* protected by d_lock */
spinlock_t d_lock; /* per dentry lock */
- struct inode *d_inode; /* Where the name belongs to - NULL is
- * negative */
+ int d_mounted;
/*
* The next three fields are touched by __d_lookup. Place them here
* so they all fit in a cache line.
@@ -93,6 +96,8 @@ struct dentry {
struct dentry *d_parent; /* parent directory */
struct qstr d_name;
+ struct inode *d_inode; /* Where the name belongs to - NULL is
+ * negative */
struct list_head d_lru; /* LRU list */
/*
* d_child and d_rcu can share memory
@@ -110,7 +115,6 @@ struct dentry {
#ifdef CONFIG_PROFILING
struct dcookie_struct *d_cookie; /* cookie, if any */
#endif
- int d_mounted;
unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /* small names */
};
-
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